The invention relates to improvements in rotational dryer drums of the type used for paper making machines wherein a traveling web of paper passes over the surface of a rotationally mounted drum for evaporating the moisture from the web and pertains particularly to a method and mechanism for increasing the transfer of heat from the steam within the drum to the drum shell and decreasing the insulating effect of the rimming condensate which forms against the inner surface of the shell periphery.
In a paper making machine the drying section generally includes a series of cylindrical dryer drums which are rotationally mounted and which are spaced so that the paper web is threaded sinuously over the drums to be in engagement with the surface of the drum or to be carried by a felt over the surface of the drum so that the web is heated for evaporating the moisture therefrom. In a high speed paper making machine, the dryer section consisting of a large number of these dryer drums takes up the largest amount of machine room floor space, and is a critical part of the machine. In speeding up the operation of the machine, it is necessary to increase the number of drums, and the space required for such drums is a limiting factor in designing the speed of the paper making machine. It is imperative to obtain the maximum drying effect for a web during its time of contact with the dryer drums and it is, therefore, imperative that the drum be heated to the maximum permissible temperature and that this heat be maintained constant and uniform during operation. The amount of heat energy absorbed by the drying web is very considerable, and the requirements of high performance, quality of production, and efficiency of production, demand that there be maximum heat energy transfer from the steam which is used to heat the dryer drum to the shell which contacts the felt carrying the paper web. A physical phenomena of operation which limits heat transfer from the steam within the drum is the formation of condensate within the drum which condensate forms a layer on the inner surface of the drum shell having insulating effect and limiting the conductive heat transfer from the steam to the metal of the shell. Efforts have been substantial to reduce the effect of this rimming insulating layer of condensate and condensate removal pipes and other devices have been provided to maintain the layer of condensate at a minimum thickness, but it is impossible to eliminate the layer completely, and even a very thin layer of condensate will have an insulating effect. An example of one such effort to reduce the insulating effect of the condensate is shown in U.S. Pat. No. 3,724,094 which is an extension of efforts illustrated and described in U.S. Pat. No. 3,217,426. These patents employ the expedient of providing axially extending bars along the inner surface of the drum shell so as to cause a surface wave to form or a sloshing of the condensate in the spaces between the bars, and the above patents show attempted improvements on a concept of providing this wave or sloshing effect as disclosed in a periodical referred to in U.S. Pat. No. 3,724,094, "Das Papier", Vol. 14 No. 10a, of Oct. 1960 (pages 600-609). The efforts exemplified by the above patents and article meet with disadvantages in that the physical space required for the bars prevents positioning condensate removal devices in very close running relationship with the inner surface of the shell. Other disadvantage is encountered in that the sloshing of the condensate will have an adverse effect on the rotational operation of the drum and will cause an erratic power demand with cascading of the water. This is especially disadvantageous because it is essential that the dryer drums be operated at uniform speed and nonuniform power requirements will adversely affect attempts to maintain this uniform speed.
It is accordingly an object of the present invention to provide an improved method and mechanism for decreasing the heat transfer retarding effect of the layer of condensate which forms on the inner surface of a rotational dryer drum.
A further object of the invention is to provide a means and method to decrease the insulating and heat retarding effect of the layer of condensate on the inner surface of a dryer drum which does not adversely affect or interfere with any other factors of performance of the drum, which embodies mechanism which is inexpensive and simple to construct and which can be mounted in new drums or utilized in existing drums, which is self-cleaning and does not require continual attention or adjustment which is easy and inexpensive to mount, which does not interfere with condensate removal, which can operate without contact with the drum so that it does not increase power requirements and which does not cause an erratic power demand for rotation of the drum.
A still further object of the invention is to provide a unique means for increasing heat transfer from the steam within the drum to the outer surface which uniquely and unexpectedly operates with the existence of a layer of condensate within the drum and yet reduces its heat transfer retarding effect so that calculated insulation properties of the layer of condensate as computed by existing formulas do not apply.
Other objects, advantages and features, as well as equivalent methods and structures which are intended to be covered herein, will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiments in the specification, claims, and drawings, in which: